The present invention relates to the production of asphalt; more particularly, it relates to methods for producing high quality, homogeneous asphalt from hydrocarbon feedstocks not normally suited for asphalt production.
Asphalts are defined as dark brown to black cementitious materials, solid or semisolid in consistency, in which the predominating constituents are bitumens which occur in nature or are obtained in petroleum processing (ASTM Designation D8). Asphalts comprise primarily very high molecular weight hydrocarbons called asphaltenes, with lesser amounts of resins, oils, waxes and other organics and inorganics.
Homogeneous asphalts, as compared to heterogeneous asphalts, are considered "purer" asphalts in that they are more uniformly constituted and generally have fewer impurities such as minerals and unwanted carbonaceous matter, such as carboids and carbenes or cracked hydrocarbons, which may affect the stability, ductility and temperature susceptibility of the asphalt. A homogeneous asphalt is, therefore, a more stable and preferred asphalt. Homogeneity is determined by the well-known "Oliensis" spot test.
While some asphalts may be utilized as taken from the ground, the overwhelming percentage are produced as products from petroleum processing. The bulk of asphalt production comes from the distillation of crude oils. These asphalts are called "straight-run" asphalts, and are obtained as residues from the physical separation of the heavier molecular weight, lower boiling point constituents of crudes from the lighter, more volatile parts. Crude oils comprising greater than 30% asphaltenes can generally be distilled in an atmospheric distillation unit to produce useable asphalts as the bottoms residue. The residues from crudes comprising less than 30% asphaltenes, however, are not immediately suitable and require further processing.
Residues from the distillation of crude oils comprising 15% to 30% asphaltenes can generally be redistilled a second time, usually through a vacuum or steam distillation unit, to remove more of the lighter materials to therby produce asphalts. Residues from the distillation of crude oils comprising less than 15% asphaltenes may be run through a propane deasphalting unit whereby small amounts of asphalt are precipitated from the residue by the treatment with propane under controlled conditions.
Straight-run asphalts are generally used in the paving industry where they serve as binders in paving mixes and as bases in liquid asphalts used as seal coatings, surface treatments, road mixes and soil stabilizers.
Some asphalts are produced by air-blowing fluid, semisolid or solid "straight-run" asphaltic residues produced by the various distillation processes. Air-blowing is a process by which air or other oxygen containing gases are passed through the asphaltic residues of elevated temperatures to harden them to a desired quality. It is believed that the oxyen chemically reacts with the hydrocarbons through the dehydrogenation and condensation of unsaturated linkages within the hydrocarbon molecules and polymerization of some of the lower molecular weight molecules to increase their molecular weights and generate more asphaltenes. These chemical changes alter the rheological properties of the asphalt to produce a harder material. Air blown asphalts are primarily used in the roofing and asphalt specialties industries.
Smaller amounts of asphalt are produced synthetically from a variety of processes, most particularly from the thermal cracking of heavier hydrocarbons. Some asphalts are also produced from the blending of two or more residues to produce an intermediate grade of asphalt, such as the blending of the resiny residues from the refinery processing of lubricating oils and the hard asphaltic residues from catalytic cracking-feed decarbonization operations to produce a desired intermediate grade of asphalt. The synthetic and blended asphalts are primarily used in the asphalt specialties industries.
The processes described above generally require feedstocks having a high asphaltene content, greater than 10% by weight, and a low wax content. Few processes, however, are capable of producing a high quality, homogeneous asphalt from crude oils or residues not normally suited for asphalt production, such as those that are paraffinic, non-asphaltic or low asphaltic, and waxy. Paraffinic hydrocarbons characteristically comprise lighter molecular weight molecules, while asphalts comprise primarily heavier molecular weight asphaltenes. Nonasphaltic or low asphaltic oils and residues also lack the main asphalt constituents, asphaltenes, and other heavier molecular weight molecules more easily convertible into asphaltenes. Waxy oils and residues produce brittle and otherwise unsuitable asphalts.
One process whereby low asphaltene content petroleum residues, that is those with less than 10% by weight asphaltenes, may be utilized to produce asphalts is disclosed in U.S. Pat. No. 2,220,714. The process therein consists of air-blowing distillation residues having less than 10% by weight asphaltenes and sulfur for 0.5 to 10 hours at a temperature of from 300.degree. F. to 550.degree. F., then removing the volatile oils from the air-blown residue and blending the remainder with a high boiling point hydrocarbon rich in resins to produce the desired asphalt. The quantity of sulfur used therein may vary from 0.5% to 5% by weight, but is preferably from 1% to 3% by weight.
U.S. Pat. No. 4,440,579 also discloses a process whereby less desirable crude stocks may be used to produce asphalts. The process therein consists of air-blowing petroleum residues in the presence of an organic sulfonic acid catalyst for a period of from 0.5 to 12 hours at a temperature of 400.degree. F. to 550.degree. F. The quantity of organic sulfonic acid catalyst employed may vary from 0.25% to 10% by weight.
With the present threats of oil shortages and resultant research into production of higher value fuels from less desirable feedstocks, including the asphaltic residuums, the present feedstocks and supply of asphalt are dwindling. In sharp contrast, the demand for asphalt is ever on the rise due to increased road and housing construction. Because of the dwindling supply and increasing demand, new feedstocks for the production of asphalt, new methods for producing asphalt and new supplies of asphalt in general are required to meet this impending need. It is, therefore, an object of the present invention to provide a method whereby high quality, homogeneous asphalts may be produced from alternate hydrocarbon feedstocks, namely those not ordinarily suited for asphalt production.